How to create a city game?

I know how to create, but I want to know how to do it:

Create a system of coins, for when we are in the store and buying a house, lose the money that the house cost and how do I fzer to prohibit putting a house on a site already occupied?

have a global variable money and subtract the cost of a house from it when the player buys one..

check if a house is already built in that spot by checking if creation square is overlapping a house, could work..

A lot of it depends on how you are planning to set upyour graphics and your interface, but if you already have trouble answering these relatively simple questions, you might want to start with an easier game first..

Ok, thank you :/!